The printing industry has experienced an evolution resulting from efforts to speed up the printing process and at the same time to lower costs. The conventional film transfer to plate scheme simply exposes light sensitive photographic film with light from a low power source and then uses the processed film to transfer the image to a plate. More efficient plate imaging methods have led to the direct imaging of the plates themselves. Although still using relatively low power light sources, these methods require complex plate composition and processing chemistry, and hence, are expensive. In a drive to reduce plate costs, the industry has moved to high power laser light sources as a means of thermally producing the image on the plate. In such applications, use is made of inexpensive coatings that require little or no processing to condition the plate surfaces for printing.
The photo-sensitive emulsions appropriate for coating conventional printing plates are based on photo-polymerization reactions, which require high levels of ultra-violet exposure. The source power required to expose a conventional plate efficiently, with a raster film recorder, is prohibitive using present day technology. A transfer medium is therefore used composed of silver-halide based emulsions which are much more sensitive to longer wavelength light and require significantly reduced levels of exposure to sensitize. After the image has been generated on the transfer medium, it is used as a photographic mask and copied by contacting it to the printing plate and providing exposure from a high intensity ultra-violet flood lamp.
Lasers are the favored light source for many raster recording devices because of their inherent high brightness, but they are limited to known lasing materials which impose a number of design restrictions, such as the choice of available wavelengths. In particular, ultra-violet laser sources are much more difficult to manufacture, and are considerably more costly than longer wavelength lasers. Presently, semi-conductor lasers are the most commercially viable laser, in terms of cost per unit emitting power. However, they are only capable of emitting wavelengths in the near infra-red to red portion of the optical spectrum. For this reason, printing plate manufacturers have recently developed printing plates based on thermally induced material changes that are sensitive to high power, near-infra-red (NIR) exposure instead of ultra-violet.
There are different optical system architectures used to record raster images on to flexible media. One such system is the internal drum scanning system in which a flexible medium is seated against an interior cylindrical mounting surface. A rotating optical element, usually a mirror or prism, which is disposed along the axis of the cylinder, redirects the modulated light beam radially with respect to the cylinder axis, scanning the beam along the cylinder circumference as it spins. The rotating scanner is translated by means of a mechanical carriage transport, which provides the slow scan axis of motion. Many machines in commercial production today employ this basic architecture in one form or another.
Another architecture is the external drum architecture in which a rotating drum carries a light sensitive plate or film clamped or otherwise held against its exterior surface. A writing head moves back and forth along the length of the drum and exposes pixels on the light sensitive recording medium. A major problem with such a system resides in the requirement of having to prevent or compensate for vibration of the large rotating drum. Moreover, because the drum is rotating it is necessary to stop the rotation after complete exposure of the plate or film, remove the latter, mount another and then start up the system again. The throughput of an external drum system is, therefore, relatively slow compared to an internal drum system in which the drum does not rotate.
A major advantage of the internal drum configuration resides in the fact that it does not require the large mass associated with the light sensitive medium to rotate. Consequently, large speeds in such systems may be obtained while still maintaining mechanical accuracy. In addition, the configuration does not suffer from inherent distortion as does the planar recording projection system. The beam is directed through the central axis of the lens elements, and the distance to the recording plane is maintained constant throughout the scan motion. This results in a very simple, robust and inexpensive optical system. However, a single faceted axial optical scanning element, used in an internal drum scanning system, causes the projected image at the recording plane to rotate about the optical axis as the beam scans along the cylinder circumference. If only circular symmetric, single spots are to be projected on to the recording plane this rotation effect is unimportant. Therefore, circular beam lasers are the natural choice for use with an internal drum scanning system.